The effect of pH on the early interaction of West Nile virus with P388D1 cells

The interaction between the flavivirus West Nile virus (WNV) and cells of the mouse macrophage-like cell line, P388D1, was assayed by transmission electron microscopy, by following the association of [35S]methionine-labelled virus with cells, and by using a radiobinding assay with an 125I-labelled F(ab')2 fragment of a monoclonal antibody directed against the major viral envelope surface glycoprotein. Using electron microscopy, both fusion and endocytosis were observed at pH 6.4, but at pH 8.0 only endocytosis was observed. When 35S-labelled WNV was bound to the P388D1 cell surface at 0 degrees C, less virus eluted on warming to 37 degrees C at mildly acidic than at alkaline or neutral pH values. The monoclonal antibody fragment had an increased affinity for cell surface viral E glycoprotein after prebound WNV was warmed at mildly acidic pH values. It is proposed that the warming of cell-virus mixtures at low pH results in fusion with a consequent reduction in elution of virus and an increase in the recognition of cell surface-expressed viral envelope glycoprotein by labelled antibody.